The present invention relates to an apparatus for analyzing a sample by electromagnetic irradiation and subsequently examining the particles emitted from the sample with the aid of a mass analyzer which forms part of the apparatus and which is accommodated in a vacuum chamber.
An apparatus of the above-outlined type is known and is disclosed, for example, in German Laid-Open Application (Offenlegungsschrift) No. 1,598,632. In this conventional arrangement, part of the sample is evaporated and ionized with the aid of high-energy electromagnetic radiation, in particular laser radiation, so that the location of impingement of the electromagnetic radiation can be used as an ion source for mass spectroscopy. In order to be able to extract the ions from the sample in the direction of the mass analyzer or in the direction of an electrode system disposed upstream of the mass analyzer, a suitable electrical potential difference must exist between the sample and the mass analyzer or the electrode upstream thereof. In this connection, reference is made to German Laid-Open Application (Offenlegungsschrift) No. 2,540,505. For an accurate analysis of the ion mass, it is of importance that this potential difference be the same, as closely as possible, for all ions emitted by the sample. This, for example, is no longer the case if, due to the emission of ions by the sample, the electrical potential changes at the locus of the evaporation of the sample material. The velocity of the ions reaching the mass analyzer then no longer corresponds to the applied accelerating voltage, but to the actual voltage difference between the locus of evaporation and the mass analyzer. Such a voltage difference is not known precisely and is not reproducible. Ions of the same mass can therefore enter the mass analyzer at different velocities which considerably worsens the resolution or even leads to measuring errors. The greater the energy of the laser pulse, and thus the number of ions generated, and the less the area of the sample under bombardment as well as the conductivity of the sample material, the greater such errors will become. The time required to compensate for this local charge must be short compared to the time of the actual measuring process.